Think of zoom like shooting a rifle. The closer the target, the easier it is to hit, even if you arms are swaying a bit. Now think of a target that's 100 yds away, any small movement in your body position could cause the bullet to miss the target completely. It's the same with zoom, when you have a short focal length they're sharp, when you extend the zoom all the way, any movement can cause noticeable fuzziness. A good rule of thumb is the minimum shutter speed you can use is 1 over your focal length. I'll use my 28-300mm as an example again. At a 28mm focal length (wide angle) my minimum shutter speed I could use would be 1/28 (realistically 1/30 since 1/28 doesn't exist) and at 300mm focal length (zoomed all the way in) my minimum shutter speed would be 1/300 (realistically 1/320 since my cam doesn't do 1/300).
When dark rooms will almost always need a tripod, if you were in a room that had maybe a single lamp on, like a 60W bulb, 400 should work, if not, step up to 800. You should be shooting with the biggest aperture you can (smallest f/stop number), I'll assume you're using an 18-55 kit lens, just to be broad, and that should be as wide as possible (18mm) and a shutter speed of 1/20. Adjust your ISO as low as you can, while still being able to properly expose the shadows (or at least the lowlights) of the composition.
IS lens have moveable lens groups that moves with the shaking of the lens and also have accelerometers, thus adding to their cost. Canon IS and Nikon VR can compensate for shaking four stops better than non-IS/VR lenses. Take the "1 over focal length example" on a 50mm prime lens (non-zooming/fixed focal length). The slowest shutter you could use without a tripod and not get blur would be 1/50 (1/60 realistically), on an IS lens you could shoot the same shot with 1/4 and using a smaller aperture to get more of the frame in focus (less depth of field).
EF and EF-S. You can use both types of lens if you're shooting with a Rebel (I think you are, I can't remember). EF lenses can be used on 35mm, full frame digital, and APS-C digital (Rebel), while EF-S lenses can only be used on APS-C cameras because EF-S lenses move the rear element closer to the image sensor (the -S stands for short back focus). If an EF-S lens is used on 35mm of full frame digitals it can cause vignetting (similar to my signature V).
So to answer your question about IS/VR in zoom vs. prime, it shouldn't really matter. Zoom lenses have a range of focal lengths (28-300mm for example), while a prime is stuck at one focal length (50mm). Prime lenses are often much cheaper and have larger apertures (see Canon 50mm f/1.2L vs. my Tamron 28-300mm which has a maximum aperture of f/4 at the same focal length (50mm)).
Hope this helps clear some things up, but I may have confused you more in the process.